This invention relates to broadband analog video transmission.
Rather than transmit broadband video signals over standard coaxial cables, it is usually more economical to transmit over unshielded twisted wire pairs which are used for analog voice transmission in telecommunications systems. Such a transmission system would allow broadband signals to be brought to the desk using wiring which is already available in the physical structure. (See, e.g., Systimax.RTM. Premises Distribution System Components Guide, AT&T Doc. No. 3726C (Dec. 1990), p. 3-10.)
U.S. Pat. No. 3,731,237 issued to Beurrier suggests use of an autotransformer with electrically coupled multiple windings for broadband transmission. Electrical coupling between the windings can be via twisted pairs. However, it does not appear that transmission of the video signals is intended to take place over unshielded twisted wire pairs.
U.S. Pat. No. 4,885,747 issued to Foglia utilizes a transformer with three windings to transmit both broadband and baseband video signals over shielded twisted wire pairs. Again, no transmission over unshielded twisted wire pairs is contemplated.
U.S. Pat. No. 5,010,399 issued to Goodman et al. shows an isolation transformer with three windings as part of a coupling network which modulates a baseband or broadband video signal so it can share the same unshielded twisted wire pair as voice signals.
U.S. Pat. No. 5,283,789 issued to Gunnarsson et al. utilizes an isolation transformer and an inductive network to couple both baseband digital data and broadband video signals onto a single unshielded twisted wire pair.
A significant problem in attempting to transmit broadband signals on unshielded twisted pair cables is the development of noise in the form of common-mode currents which can travel down the wire pairs causing radiated emissions exceeding electromagnetic interterence (EMI) standards. It is desirable, therefore, to provide a circuit which will inexpensively limit the appearance of common-mode currents in broadband video transmission.